1. What Gear Is Needed?
If you plan on taking your none-waterproof camera underwater you'll want to fix a wide-angle or even fisheye lens to it and secure it in waterproof housing. There's also several waterproof compacts available which don't need housing as they're designed to survive a swim to certain depths.
2. How Well Do You Know Your Camera?
Finding and changing your camera's settings underwater, particularly if your camera's in waterproof housing, can be really hard work so if you don't want to miss a shot, set your camera up before you put the camera in the water. Fish move quickly, too so if you're fiddling about with your camera, chances are you'll miss the opportunity to capture your shot. Once you've taken your photos do remember your camera, even if it is waterproof, isn't a great fan of salt water so wipe and dry it down.
3. What Is The Weather/Conditions Like?
You need water that's clear as possible so avoid underwater photography on choppy days when visibility can be poor due to sand and other debris getting churned around in the swell. Light's less even and there's not that much of it underwater so you'll need higher ISOs and wider apertures. You'll also want to put as little distance between you are your subject as you can, otherwise you'll have blurry shots where you can't make out if you've captured a fish or a piece of seaweed. Water has a habit of refracting light which can mean your subject appears closer and larger than it really is, too.
4. Is It Possible To Shoot Near The Surface?
If you can work closer to the surface do as colours are more vibrant as light is less diffused and the reflections on it, like when working above water, can add extra interest to your underwater shots. When you do venture into deeper water do remember to secure your camera correctly after all, you don't want to watch it sink away from you.
5. Is Flash Needed?
You can use your camera's built-in flash to add more light to your shot but it won't work if you're not working close to your subject as it'll light up any tiny pieces of matter floating around which can cause blob-like shapes to appear around your image. Instead of using the built-in flash try taking a more powerful strobe underwater with you as you can move these around the scene so the blobs of light won't make an appearance. You'll also be able to create different effects, adding more light to some areas and creating shadows in others.
Unless you're close to the surface where it's a little lighter, you'll most likely need flash to freeze the movement of the fish and plants you're photographing. Flash is also handy when you're taking a macro shot of coral.
6. Are White Balance Adjustments Needed?
Unless you change your white balance your underwater shots will have the familiar blue/green cast to them. Adding flash can help remove it but the best way to control it is by manually setting the white balance. All you have to do is take a reading off something light and when you alter your depth, take another reading as variables change.
7. Is There A Method For Approaching Fish Without Scaring Them?
If you have the time to learn a little about the fish species that will be around the place you're visiting do as knowing which species will be more willing for you to get close and understanding the behaviour of the more shy species will help you produce more, frame-filling, interesting results.
If you didn't plan on venturing out under the waves just move in gradually when you spot a fish you'd like to photograph and see how it reacts to you. Don't suddenly start flapping your arms or kicking your legs though as this will just cause them to turn around and head to safety. The bubbles you make from breathing out can also scare them but we're not suggesting you hold your breath! Just try and breath out in a different direction to where the fish is. Weights and a buoyancy compensator can help you keep still but these are generally used by more serious underwater photographers.
8. What About Backgrounds?
Where possible, keep the background plain and clutter free after all, a lot of creatures are designed to camouflage with their surroundings but you don't want your friends to be playing 'spot the underwater creature' when you show them your shots. The best plain background you'll find is the huge amount of water above your head. To get it in shot simple shoot from a low angle. This will also help your subject look more dominant.
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